1. Field of the Invention
The present invention relates to a semiconductor device with a negative voltage regulator, and more particularly, to a semiconductor device with a negative voltage regulator utilizing triple-well NMOS transistors.
2. Description of the Prior Art
There are a lot of applications that utilize regulators for tasks of regulating voltages. Many designs and patents of regulators have been developed for improving the performance of regulator circuits. One of the examples is U.S. Pat. No. 6,600,692, Semiconductor Device with a Voltage Regulator” to Tanzawa, which is included herein by reference.
Many applications require circuits that can boost up an input power supply DC voltage to a higher DC voltage used for specialized operations. The reason for the voltage boost up is that often only standardized power supply voltages are available for supplying power to electronic circuits. However, sometimes there are situations where a circuit needs a higher voltage than one available from the associated power supply. In addition, other circuits even require a negative voltage though only positive voltages from a power supply are available. One example of such a circuit is an electrical erasable programmable read only memory (EEPROM), typically termed in the art as “flash memory”. A flash memory may require a negative voltage to perform erase operations. However, there are few achievements in regulating negative voltages. Techniques for regulating positive voltages, such as illustrated in U.S. Pat. No. 6,600,692 are not applicable to regulating negative voltages. In general, a negative pump is often utilized to generate a negative voltage. Please refer to FIG. 1. FIG. 1 is a block diagram of a prior art negative voltage generator 100. The negative voltage generator 100 includes an oscillator 110 and a negative pump 120. The oscillator 110 outputs its output to the negative pump 120, and then a negative voltage VOUT1 is output from the negative pump 120. Please refer to FIG. 2. FIG. 2 is a block diagram of a prior art negative voltage regulator 200. The negative voltage generating circuit part includes an oscillator 210 and a negative pump 220 the same as the circuit in FIG. 1. The negative voltage regulating circuit part includes an AND gate 230, a voltage potential divider 240 and a comparator 250. Vref21 and Vref22 are two reference voltages. R21 and R22 are two voltage dividing resistors. Compared to the unregulated voltage VOUT1 in FIG. 1, the voltage potential divider 240 divides the output voltage of the negative pump 220, VOUT2 and the reference voltage Vref21, and then inputs the voltage VFEBK2 generated in the voltage division into the comparator 250 to be compared with the reference voltage Vref22. The output of the comparator 250 and the output of the oscillator 210 are input to the AND gate 230, and the output of the AND gate 230 is then input to the negative pump 220. Thereby a regulation loop is formed, and the voltage VOUT2 is a regulated negative output voltage.
For circuits that require high precision, the conventional negative voltage regulator 200 illustrated in FIG. 2 is not ideal. The operation of the conventional negative voltage regulator 200 illustrated in FIG. 2 is described as below. When the potential of the voltage VOUT2 is lower than a predetermined potential, the feedback voltage VFEBK will be pulled down and the output of the comparator 250 is made digital 0 (low potential). The output of the AND gate 230 is made digital 0, hence the negative pump 220 stops charging along with the oscillator 210 and pulls up the potential of the voltage VOUT2. Contrarily, when the potential of the voltage VOUT2 is higher than the predetermined potential, the feedback voltage VFEBK will be pulled up and the output of the comparator 250 is made digital 1 (high potential). Therefore the negative pump 220 charges along with the oscillator 210 and then decreases the potential of the voltage VOUT2. The regulation as described above is limited by the comparison range of the comparator 250 and the AND gate 230, and is similar to digital feedback regulation. The potential of the regulated voltage VOUT2 still suffers significant ripple. In addition, the performance of the conventional negative voltage regulator 200 does not sufficiently meet the requirements of circuits that need to utilize negative voltages.